Since excellent fatigue properties are desired for machine structural components such as automobile gears, surface hardening treatment is usually performed when manufacturing such components. Examples of well-known surface hardening treatment include carburizing treatment, induction quench hardening, and nitriding treatment.
Among these, in carburizing treatment, C is immersed and diffused in high-temperature austenite region and a deep hardening depth is obtained. Therefore, carburizing treatment is effective in improving fatigue strength. However, since heat treatment distortion occurs by carburizing treatment, it was difficult to apply such treatment to components which require severe dimensional accuracy from the perspective of noise or the like.
Further, in induction quench hardening, since quenching is performed on the surface layer part by high frequency induction heating, heat treatment distortion is generated, and therefore results in poor dimensional accuracy as in the case with carburizing treatment.
On the other hand, in nitriding treatment, surface hardness is increased by immersing and diffusing nitrogen in a relatively low temperature range at or below the Ac1 transformation point, and therefore there is no possibility of heat treatment distortion such as mentioned above. However, there were problems that the treatment requires a long time of 50 hours to 100 hours, and then it is necessary to remove brittle compound layers on the surface layer after performing the treatment.
Therefore, nitrocarburizing treatment in which treatment is performed at a treatment temperature almost equal to nitriding treatment temperature and in a shorter treatment time was developed, and in recent years, such treatment has been widely used for machine structural components and the like. During this nitrocarburizing treatment, N and C are simultaneously infiltrate and diffused in a temperature range of 500° C. to 600° C. to harden the surface, and the treatment time can be made half of what is required for conventional nitriding treatment.
However, whereas the above mentioned carburizing treatment enables to increase the core hardness by quench hardening, nitrocarburizing treatment does not increase core hardness since the treatment is performed at a temperature at or below the transformation point of steel. Therefore, fatigue strength of the nitrocarburized material is inferior compared to the carburized material.
In order to improve the fatigue strength of the nitrocarburized material, quenching and tempering are usually performed before nitrocarburizing to increase the core hardness. However, the resulting fatigue properties cannot be considered sufficient. Furthermore, this approach increases manufacturing costs and reduces mechanical workability.
In order to solve such problem, JPH0559488A (PTL 1) proposes a steel for nitrocarburizing which enables to obtain high bending fatigue strength after nitrocarburizing treatment by containing Ni, Al, Cr, Ti, etc. in steel.
Regarding this steel, by performing nitrocarburizing treatment, the core part is age hardened by Ni—Al based or Ni—Ti based intermetallic compounds or Cu compounds, while in the surface layer part, for example, Cr, Al, Ti nitrides or carbides are precipitated and hardened in the nitride layer, to improve bending fatigue strength.
JP200269572A (PTL 2) proposes a steel for nitrocarburizing which provides excellent bending fatigue properties after nitrocarburizing treatment by subjecting a steel containing 0.5% to 2% of Cu to extend forging by hot forging, and then air cooling to obtain a microstructure mainly composed of ferrite with solute Cu, and then precipitating the Cu to contribute to precipitation hardening during nitrocarburizing treatment at 580° C. for 120 minutes, and further use precipitation hardening by carbonitrides of Ti, V and Nb with precipitation hardening by Cu.
JP2010163671A (PTL 3) proposes a steel for nitrocarburizing obtained by dispersing Ti—Mo carbides, and further dispersing carbides containing at least one of Nb, V, and W.